Packing materials for liquid chromatography (LC) are generally classified into two types: organic materials, e.g., polydivinylbenzene, and inorganic materials, e.g., silica.
As stationary phases for HPLC, silica-based materials result in columns that do not show evidence of shrinking or swelling and are mechanically strong. However, limited hydrolytic stability is a drawback with silica-based columns, because silica may be readily dissolved under alkaline conditions, generally pH>8.0, leading to the subsequent collapse of the chromatographic bed. Additionally, the bonded phase on a silica surface may be removed from the surface under acidic conditions, generally pH<2.0, and eluted off the column by the mobile phase, causing loss of analyte retention.
On the other hand, many organic materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase pH. However, organic chromatographic materials generally result in columns with low efficiency, leading to inadequate separation performance, particularly with low molecular-weight analytes. Furthermore, many organic chromatographic materials shrink and swell when the composition of the mobile phase is changed. In addition, most organic chromatographic materials do not have the mechanical strength of typical chromatographic silica.
In order to overcome the above-mentioned deficiencies while maintaining the beneficial properties of purely organic and purely inorganic materials, others have attempted to simply mix organic and inorganic materials. For example, others have previously attempted to produce such materials for optical sensors or gas separation membranes that are mixtures of organic polymers (e.g., poly(2-methyl-2-oxazoline), poly(N-vinylpyrrolidone), polystyrene, or poly(N,N-dimethylacrylamide) dispersed within silica. See, e.g., Chujo, Polymeric Materials: Science & Engineering, 84, 783 (2001); Tamaki, Polymer Bull., 39, 303 (1997); and Chujo, MRS Bull., 389 (May 2001). These materials, however, were not useful for any liquid based separation application because they are translucent and non-porous. As a result, these materials lack capacity as a separation material.
Still others have attempted to make materials that have inorganic and organic components covalently bound to each other. See, e.g., Feng, Q., J. Mater. Chem. 10, 2490-94 (2000), Feng, Q., Polym. Preprints 41, 515-16 (2000), Wei, Y., Adv. Mater. 12, 1448-50 (2000), Wei, Y. J. Polym. Sci. 18, 1-7 (2000). These materials, however, only contain very low amounts of organic material, i.e., less than 1% C, and as a result they function essentially as inorganic silica gels.
Furthermore, these materials are non-porous until they are ground to irregular particles and then extracted to remove template porogen molecules. Accordingly, it is not possible to make porous monolithic materials which have a useful capacity as a separation material. Irregularly-shaped particles are generally more difficult to pack than spherical particles. It is also known that columns packed with irregularly-shaped particles generally exhibit poorer packed bed stability than spherical particles of the same size. The template agents used in the synthesis of these materials are nonsurfactant optically active compounds, and the use of such compounds limits the range of porogen choices and increases their cost. The properties of these materials make them undesirable for use as LC packing materials.